[gdal-dev] Re: can ogr2ogr split polygons that cross 180 degree longitude

[Hermann Peifer]

Even,

Thanks for the possible algorithm.... but to be honest: some sort of 
command line switch would be /somewhat more handy/, for non-programmers 
like me.

Hermann

Even Rouault wrote:
> Selon Hermann Peifer <peifer at gmx.eu>:
>
> I think this should be prefectly doable with some code/script just using OGR
> API.
>
> Here's a possible algorithm :
>
> 1) Create a new empty WGS84 shapefile
> 1) Loop over features in the source shapefile (the one after reprojection to
> WGS84)
> 2a) If the polygon has points with longitudes both near 180° and near -180°,
> create a duplicate temporary polygon and add 360° to the negative longitudes.
> Compute the interescection with Geometry.Intersect (you need GEOS support) of
> that polygon with POLYGON((0 90,180 90,180 -90,0 -90,0 90)) and with
> POLYGON((180 90,360 90,360 -90,180 -90,180 90)). For the second intersection,
> substract 360° to the longitudes in order to get again longitudes in the
> [-180,180] range. Finally, write 2 new features into the output shapefile, or
> better, just one with the geometry being a multipolygon with the 2 parts.
> 2b) Otherwise, just copy the original feature into the output shapefile
>
> Best regards,
> Even
>
>   
>> Eric,
>>
>> I can't help much apart from confirming that the same happened to me
>> when trying to reproject Russia from LAEA to WGS84.
>>
>> My "solution" was to ask a colleague, who is using ESRI products to do
>> me a favour... :-(
>>
>> Hermann
>>
>>
>> Eric Muller wrote:
>>     
>>> I am trying to reproject  shapefiles that represent Russia (from
>>> http://wagda.lib.washington.edu/DATA/geography/world/russia/download.html)
>>> from the original:
>>>
>>>     The map is Albers equal-area projection Clarke spheroid of1866 with
>>>     the first standard parallel 46 00 0.000, the second standard
>>>     parallel 73 00 0.000, the central meridian 84 00 0.000. The latitude
>>>     of the projection's origin is 60 00 0.000.  Units -- meters.
>>>
>>> to lat/long WGS84. I use the following command:
>>>
>>> ogr2ogr \
>>>     -s_srs '+proj=aea +ellps=clrk66 +lat_1=46.0 +lat_2=73.0 +lat_0=60.0
>>> +lon_0=84.0 +units=m +no_defs' \
>>>     -t_srs '+proj=latlong +ellps=WGS84 +datum=WGS84 +no_defs' \
>>>     rs_polbnda_${f}.shp rus${f}.shp
>>>
>>> I would like polygons that cross the 180 degree longitude to be split.
>>> It there a way to ask ogr2ogr to do that? What is the recommended way to
>>> do it, if not?
>>>
>>> Thanks,
>>> Eric.
>>>
>>>
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>
>